Systems for processing semiconductor wafers typically process the wafers in loads or batches to maximize efficiency and throughput. Each batch of the wafers is mounted on a single, portable carrier, sometimes referred to as a cassette, wherein the wafers are effectively disposed in a stacked arrangement. After the wafers are loaded into the cassette, the cassette is placed on a platform-like, cassette stage within an enclosed housing referred to as a load lock chamber. After the cassette is installed in the chamber, a loading door of the chamber is closed and a vacuum is drawn within the chamber, at which point the wafers are ready to undergo processing. Typically, the wafers are then serially removed from the cassette and transferred to other chambers for appropriate processing.
After closing the load lock chamber loading door, it is not readily apparent to the human operator whether a cassette is present in a particularly chamber. Therefore, means have been devised in the past for sensing the presence of a cassette within the chamber. One form of known sensor comprises an optically based sensor which includes portions that extend into the interior of the chamber, in close proximity to the top surface of the cassette. This sensing arrangement merely detects whether a cassette is present, but does not provide any information as to the orientation or position of the cassette on the cassette stage. In some cases, the cassettes are sometimes placed upon the cassette stage such that the cassette is cocked or tilted slightly, i.e., the longitudinal axis of the cassette is not vertically aligned. Cassettes which are placed on the stage so as to be off axis can result in misfeeding or jamming of the transfer mechanism which serially removes the wafers from the cassette. Misfeeding or jamming naturally results in down time of the system, and thus less throughput, and can even reduce yield where wafers are damaged as a result of the jamming.
The prior art wafer sensing arrangement is also undesirable because of the intrusion of the optical sensor deep into the load lock chamber. The sensor is not only exposed to the atmosphere within the chamber, but is subject to damage or breakage since it tends to interfere with the installation or removal of a cassette within the chamber.
The present invention is intended to overcome each of the deficiencies of the prior art discussed above.
Accordingly, it is a primary object of the present invention to provide apparatus for detecting the presence and correct orientation of a wafer carrying cassette on a support surface within a cassette housing forming part of a semiconductor processing system.
A further object of the present invention is to provide apparatus as described above, which assures axial alignment of the cassette within the cassette housing, thereby preventing misfeeding or jamming of feed mechanisms which transfer wafers from the cassette to downstream processing stations.
A still further object of the present invention to provide apparatus as described above, which obviates the need for mounting sensing electronics within the housing.
Another object of the invention is to provide apparatus as aforesaid, which employs a simple, very reliable mechanical assembly for sensing whether the cassette is properly oriented on a support surface.
These, and further objects of the invention will be made clear or will become apparent during the course of the following description of a preferred embodiment chosen to illustrate the invention.